Coating of interior surfaces of pipe

ABSTRACT

THE INTERIOR SURFACES OF PIPE ARE COATED BY PNEUMATICALLY PROPELLING THROUGH THE PIPE A COATING LIQUID CONFINED BETWEEN A COATING APPLICATOR AND A RETAINING PLUG USING A COATING APPLICATOR CONSTITUTED BY A CYLINDRICAL SLEEVE WHICH IS FORMED WITH EXTERNAL LONGITUDINAL GROOVES AND WHICH IS FLEXIBLE AND RESILIENT TO ACCOMMODATE IMPERFECTIONS IN THE PIPE AND WHICH IS BIASED AGAINST THE INTERIOR OF THE PIPE BY MEANS OF AN INFLATED BOOT. TO FACILITATE FLOWOUT AND TO AVOID RUNNING AND SAGGING, THE LIQUID COATING IS MADE THIXOTROPIC AND TO ELIMINATE THE NEED FOR REMOVING LARGE AMOUNTS OF SOLVENT, THE COATING LIQUID IS ESSENTIALLY SOLVENT FREE AND FORMULATED TO CURE UPON EXPOSURE TO THE GAS USED FOR PNEUMATIC PROPULSION AND IS PREFERABLY MADE TO BE CURED BY CONTACT WITH ATMOSPHERI MOISTURE.

g- 1971 D. w. HONEYCUTT, JR 3,598,636

COATING OF INTERIOR SURFACES OF PIPE Filed June 1'7, 1968 2 Sheets-Sheet 1 INVENTOR DEWITT W. HONEYCUTT, JR.

JWZ

ATTORNEYS g- 1971 D. w. HONEYCUTT, JR 3,593,636

COATING OF INTERIOR SURFACES OF PIPE Filed June 17, 1968 2 Sheets-Sheet 2 INVENTOR DEWITT W. HONEYCUTT,

$wWWM ATTORNEYS United States Patent Oihce Us. (:1. 117-95 7 Claims ABSTRACT OF THE DISCLOSURE The interior surfaces of pipe are coated by pneumatically propelling through the pipe a coating liquid confined between a coating applicator and a retaining plug using a coating applicator constituted by a cylindrical sleeve which is formed with external longitudinal grooves and which is flexible and resilient to accommodate imperfections in the pipe and which is biased against the interior of the pipe by means of an inflated boot. To facilitate flowout and to avoid running and sagging, the liquid coating is made thixotropic and to eliminate the need for removing large amounts of solvent, the coating liquid'is essentially solvent free and formulated to cure upon exposure to the gas used for pneumatic propulsion and is preferably made to be cured by contact with atmospheric moisture.

The present invention relates to the coating of pipe utilizing a coating applicator which is forced through the pipe utilizing gas, preferably air, under pressure. In this invention the coating applicator is an inflated element and the coating is a self-hardening system activated by atmospheric moisture.

The invention is particularly directed to the deposition on the interior surfaces of pipes of relatively thick uniform coatings which will provide appropriate and relatively uniform protection for all of the exposed interior surfaces of the pipe. Nonetheless, the coating applicator must be able to accommodate lack of uniformity in the interior of the pipe as may occur when the pipe includes a curved section, or where there is present some minor obstruction such as collars, joints or welds, or where the pipe internal diameter varies somewhat.

The invention is also especially directed to the deposition of coatings which cure in situ, and this is particularly difiicult since the material which is deposited must remain fluid for long periods of time during application and where it is not feasible to heat or treat the wet coatings which are deposited or to remove large amounts of solvents therefrom.

In accordance with the invention, a liquid coating material is confined within a pipe between a retaining plug and a coating applicator and compressed air or other gas is forced into the pipe behind the applicator to move the plug and the applicator as a unit with the coating liquid confined therebetween through the pipe. While the foregoing is broadly known, the invention proposes to improve the practicality of the system by utilizing as the coating applicator a flexible and slightly resilient cylindrical sleeve having external longitudinally extending grooves in order that the coating liquid can be deposited via the grooves with a uniform circumferential distribution as the sleeve is forced through the pipe. The sleeve is open at its rear end to receive an inflatable boot which is inflated to pneumatically bias the sleeve against the interior of the pipe. In this way the composite applicator can deposit a relatively uniform amount of coating liquid controlled primarily by the size and cross-section of the grooves and still accommodate curved pipe, minor internal obstructions and non-uniform internal pipe diameter.

Another important aspect of the invention is the utilization as the coating material of a curable liquid which is 3,598,636 Patented Aug. 10, 1971 essentially free of organic solvents and which cures when exposed to the gas which is employed to pneumatically propel the plug and the applicator as a unit through the pipe. The coating liquidconfined between the pipe and the applicator remains out of contact with the propelling atmosphere and remains in its free flowing liquid condition. On the other hand, as soon as portions of the liquid coating pass through the longitudinal grooves in the surface of the cylindrical boot used as the coating applicator, it comes into contact with the propelling atmosphere en'- abling cure to take place. While various types of essentially solvent-free liquid coating compositions may be employed, the invention preferably employs a liquid polyepoxide in admixture with ketimine curing agent which breaks down upon exposure to atmospheric moisture releasing a small proportion of volatile ketone and an amine which is capable of curing the liquid polyepoxide under the ambient temperature conditions prevailing in the pipe. Additionally, the coating liquid is preferably formulated to include thickening agents and polar materials as needed to induce thixotropic properties so that the coating composition is of generally putty-like consistency but flows freely under shear. In this way the coating will flow through the grooves in the coating applicator and flow out to form a uniform layer, but it resists running and sagging even though applied to the interior walls of the pipe in a considerable thickness, e.g., up to about 10 mils.

As previously indicated, the coating composition which is applied in the invention does not rely on solvent removal for cure. This is because it is not practical to rely upon solvent evaporation inside a lengthy pipe when such a large amount of solvent must be removed from such a large amount of coating with so little opportunity to flush the solvent fumes out of the pipe. Solvent evaporation is particularly poor as a means to obtain rapid hardening of the applied coating and this limits coating thickness. While solvent evaporation cannot be relied upon as the primary cure mechanism, it is still desirable to purge the pipe by blowing air therethrough to assist in removing volatiles and to bring more moisture to the coating.

In the invention as soon as the coating liquid is deposited by the applicator, it contacts the propelling atmosphere and it becomes possible to rely upon the content of that atmosphere to trigger the cure of the liquid coating. In this respect, the moisture content of the atmosphere is particularly preferred because it can be used to induce relatively rapid cure in liquids which are quite stable in the absence of water.

While various liquid coatings which are moisture-curable are known, one system which may be mentioned is that of liquid polyesters which have been reacted with a stoichiometric excess of organic polyisocyanate in order to provide an isocyanate-terrninated liquid resin. The isocyanate group is reactive with atmospheric moisture to cross-link the resin and convert it from a liquid to a solid condition. The preferred moisture-activated curing coating liquid as indicated hereinbefore is a mixture of a liquid polyepoxide and a ketimine curing agent. Ketimines are known compounds produced by reacting a ketone with an amine under conditions causing dehydration. Upon contact with atmospheric moisture, the water in the atmosphere reacts with the ketimine releasing the ketone as a volatile and regenerating the amine in situ which causes the polyepoxide to cure under the prevailing ambient conditions, the amine cure of polyepoxides being well known to take place progressively at room temperature.

Any polyepoxide which is liquid at room temperature Y relatively low molecular weight diglycidyl ether of hisphenol'being especially preferred. These are particularly illustrated by a normally liquid diglycidyl ether of Bisphenol A having a molecular weight of 250 and an epoxide equivalent weight of 175.

The epoxy group is a 1,2 oxinane group and the preferred polyepoxides have an epoxy equivalency of at least about 1.4. While glycidyl ethers are preferred, these can be based on any polyhydric organic compound including aliphatic compounds such as ethylene or propylene glycol or glycerin. The term bisphenol identifies a pair of phenolic groups joined through an intervening divalent alkylene group.

The ketimines which are used can be based upon any amine or polyamine, innumerable amine curing agents being well known to the art. A particularly preferred amine is methylene dianiline.Other curing agents which may be used are illustrated by ethylene diamine, diethylene triamine, triethylene tetramine and tetraethylene pentamine. Hydroxy amines can also be used, for example, the reaction product of any of the foregoing amines with a stoichiometric deficiency of ethylene oxide. All that is required is that the amino hydrogen atoms of the amine be covered by reaction with ketone via imine formation in order that the amine might be regenerated when the deposited film containing the amine is exposed to atmospheric moisture.

While small amounts of volatile solvents may be present, e.g., up to about 10% by weight, preferably not in excess of by weight, the preferred solvents, if present should be highly volatile so as to facilitate their elimination from the system. In this respect, the breakdown of the ketimine curing agent by moisture releases the volatile ketone used in its formation and the preferred ketone is acetone or methyl ethyl ketone because of their relatively high volatility. Methyl isobutyl ketone and diethyl ketone are also useful though less preferred. The ketimine proportion is of secondary significance, but a stoichiometric relationship between the epoxy group and the amine functionality released by the ketimine is preferred. Acceptable proportions of polyepoxide and amine curing agents therefore are common knowledge in the art.

The invention will be more fully described in conjunction with the accompanying drawings in which:

FIG. 1 is a side elevation, partly in section, showing a coating applicator constructed in accordance with the invention;

FIG. 2 is a rear elevation of the structure shown in FIG. 1;

FIG. 3 is a front elevation of the structure shown in FIG. 1;

FIG. 4 is a fragmentary sectional elevation of the rear of the applicator of FIG. 1, but modified to include an air seal; and

FIG. 5 is a side elevation, partly in section, and showing liquid coating being applied in accordance with the invention.

inner inflatable boot 12. The sleeve 11 is made of flexible material illustrated by polyethylene and formed with a forward nose portion 13 and a longitudinally grooved body portion 14. The grooves extend the length of the sleeve 11 to its trailing edge 15, the grooves being individually identified at 16. As will be evident, as the applicator moves forwardly through a pipe, liquid coating will pass through the grooves 16 while the bulk of the coating will be pushed forward through the pipe by the nose 13.

The inner inflatable boot 12 is desirably made of rubber and is formed with a conical forward end 17 and a trailing end 19 which defines an opening 18. As Will be evident, the boot 12 is configured to generally fit within the 4 sleeve 11 with its trailing end 19 disposed in the vicinity of the open rear of the sleeve 11 Where it is 'clampe'd'be tween an end plate 20 and a compression ring 21 by means of nuts 22 and coacting bolts 23. As shown in FIG. 1, the end plate carries a valve 24 to permit the boot to be inflated and deflated and a pressure gauge 25 to registerthe pressure within the boot.

The shape of the grooves 16 is preferably triangular as can be seen in FIGS. 2 and 3, but the precise cross-sectional configuration is a matter of choice though a generally pointed configuration is preferred in order to m nimize the area of contact with the pipe and thereby insure that the coating will flow out and form a layer which is of generally uniform thickness.

The selection of a coating liquid possessing thixotropic properties is of evident value in minimizing the pressure required to shear the liquid coating and force it through the grooves 16. As is known, when thixotropic coatings are sheared, their viscosity is reduced momentarily but the viscous nature of the material returns with time so that while minimum movement of material adequate for flow-out can take place enabling the material which passes through the grooves to form a layer, gross overall shifts in materials as would result in running or sagging of the coating can be avoided.

FIG. 4 shows the utilization of an optional seal 30, The air seal 30 is a thin-walled sheet, e.g., about .004 inch thick, which extends between the sleeve and the boot for 3 or 4 inches where it is held in place by friction therebetween. The air seal 30 is cylindrical and extends around the entire trailing edge 15'of the sleeve 11. The propelling gas tends to force the seal 30 outwardly and this will help to smooth the coating which is applied via the grooves 16.

The overall action can be seen from FIG. 5 in which the numeral 35 identifies a pipe being coated in accord: ance with the invention with the coating liquid 36 being confined between a plug 37 of conventional construction and an applicator 10 constructed in accordance with the invention. Air containing moisture under pressure and identified by the numeral 38 to the rear of the applicator 10 causes the applicator, the plug and the coating liquid confined therebetween to move through the pipe as a unit in the direction indicated by the arrow A. It will be seen that the coating liquid 36 is kept out of contact with the air as it moves through the pipe and, in this way, the moisture-sensitive curable coating liquid 36 is maintained in free-flowing liquid condition for long periods of time as may be required in connection with the coating of the pipe. On the other hand, as soon as the coating liquid 36 passes the applicator 10 via the grooves 16 thereof the deposited coating 40 encounters the atmosphere 38 employed to propel the applicator. As previously described, this atmosphere preferably includes a desired moisture content to react with the coating liquid and cause the same to harden and cure with the expulsion of only a minimal amount of volatile solvent. The thixotropic action of the coating liquid facilitates application of the coating via the grooves 16 and flow out to a uniformed layer without running or sagging of the layer within the pipe.

It is of interest to observe that the coating 40 functions as a continuous diaphragm so that the pressure employed to advance the applicator 10 also functions to force the deposited coating 40 against the inner surface of the pipe 35 thereby improving contact of the wet coating with the interior surface of the pipe and leading to better adhesion after the coating is cured.

Operation in accordance with the invention is illustrated as follows in which it will be assumed that the pipe to be coated has an internal diameter of 4" in which case the sleeve would be 14" in length and 4" in external diameter with the thickness of the sleeve being /8" and the grooves extending a depth of X and having the configuration of right triangles. Polyethylene has been mentioned as a suitable material, but neoprene or urethane rubber may also be employed. The material should have approximately the hardness of automobile tire rubber and be slightly resilient to be expanded somewhat by the inflated boot.

As a matter of interest, the geometry of the groove arrangement provides a void volume (the space confined between the grooves and the pipe wall) of /2 the total space assuming there were no grooves so that the deposited lines of coating liquid will spread and fiow together to form a layer having a thickness of /2 the peak height. Thus, using grooves in depth, a layer 78 mils in thickness is deposited.

It is to be noted that the external diameter is measured when the sleeve has been expanded somewhat by being inflated by means of the inflated boot under an internal pressure of from 20-40 p.s.i. Precise dimensions are of secondary significance since this is accommodated by varying the pressure within the boot.

The boot has an external diameter of 3%" prior to inflation and is made of rubber of the nature of automobile tire inner tubes and having a thickness of Va". The overall length of the rubber boot is 15" and it is clamped using an end plate 3" in diameter and a compression ring having an internal diameter of 1. When the boot is assembled within the sleeve and placed under a pressure of 20-40 p.s.i., the composite applicator will fit within a 4" internal diameter pipe for propulsion therethrough using gas under pressure. Of course, it is the pressure ditference from the trailing end of the applicator to the forward end of the retaining plug which governs speed and this can be controlled as desired.

A preferred coating composition for use in the invention is provided as follows:

200 pounds of a liquid diglycidyl ether of bisphenol A having an average molecular weight of 350, and an epoxide equivalent weight of 175 has dispersed therein 200 pounds of titanium dioxide rutile and, in a separate container, 20 pounds of finely divided pyrogenic silica are dispersed in 300 pounds of additional liquid polyepoxide of the same type noted above. The silica in the presence of appropriate polar agents provides the desired thixotropic thickening action. The separate dispersions referred to above are then mixed together and 20 pounds of phenol liquified with water are added and thoroughly dispersed. 388 pounds of additional polyepoxide (the same one noted hereinbefore) are then mixed in to provide a putty-like material which flows readily under shear. More particularly the putty-like mixture has a viscosity of 280-320 poises when measured in a Brookfield viscometer at 77 F. using a No. 6 spindle rotating at 20 rpm.

The composition is employed as a two-package system with the curing agent being mixed into the composition immediately prior to insertion of the composition into the pipe between the coating applicator and the retaining plug.

In a preferred arrangement, the ketimine curing agent is constituted by a reaction product of triethylene tetramine with methyl ethyl ketone under anhydrous conditions and is used in an amount of 30 parts by weight per 100 parts by weight of liquid polyepoxide in the putty-like mixture described hereinbefore. This represents a substantially stoichiometric relationship between the amine functionality of the ketimine and the epoxy functionality of the polyepoxide.

After the coating is applied to the interior of the pipe as described to form a layer having a thickness of 7.8

mils, the pipe is purged by blowing air through the same in order to permit the ketone released by the decomposition of the ketimiine curing agent to be substantially completely removed. It is stressed however that the hardening of the coating is an incident to amine cure and not solvent release.

As a result, the coating is uniformly applied to the interior of the pipe in controlled thickness and cured in place.

The invention is defined in the claims which follow.

I claim:

1. In a method for coating the interior of pipe in which a coating liquid is confined between a coating applicator and a plug with the applicator, plug and confined liquid being pneumatically propelled through the pipe as a unit, the improvement comprising employing as said coating liquid, a curable liquid essentially free of organic solvents which is pigmented to possess thixotropic properties and which cures when exposed to the gas employed for pneumatic propulsion.

2. A method is recited in claim 1 in which the gas employed for pneumatic propulsion is air containing moisture and said curable liquid cures upon contact with moisture.

3. A method is recited in claim 1 in which said curable liquid is a liquid polyepoxide in admixture with a ketimine curing agent.

4. A method is recited in claim 3 in which said polyepoxide is a diglycidyl ether of a bisphenol.

5. A method is recited in claim 3 in which said ketimine is the reaction product under anhydrous conditions of an amine curing agent for polyepoxide with a volatile ketone.

6. In a method for coating the interior of pipe in which a coating liquid is confined between a coating applicator and a plug with the applicator, plug and confined liquid being pneumatically propelled through the pipe as a unit, the improvement comprising employing a cylindrical coating applicator formed with a multiplicity of external longitudinally extending grooves and a thixotropic coating liquid of putty-like consistency in the absence of shear.

7. A method as recited in claim 6 in which said coating liquid has a viscosity of 280-320 poises when measured in a Brookfield viscometer at 77 F. using a No. 6 spindle rotating at 20 r.p.m.

References Cited UNITED STATES PATENTS 2,445,645 7/ 1948 Stephens 118408 3,041,204 6/1962 Green 117-97 3,108,012 10/1963 Curtis 11795 3,125,464 3/1964 Harmes 118105 3,355,315 11/1967 Jorda et a1. 117-97 3,378,600 4/1968 Hodges et al 260-47X 3,397,178 8/1968 Shackelford et a1. 26047 3,401,146 9/1968 Kamal et a1. 26047 3,442,856 5/1969 Floyd 26047 ALFRED L. LEAVITT, Primary Examiner E. G. WHITBY, Assistant Examiner US. Cl. X.R. 

